Electrical measuring apparatus



1. L. CLARKE. ELECTRICAL MEASURING APPARATUS. APPLICATION mzn FEB. 10, 1920.

1,381 ,968, Patehted June 21, 1921.

[/7 venfor J56. Chico A170rne)/ LWHTED STATE5 PATENT.OFFWY,

ELECTRICAL MEASURING APPARATUS..

Specification of Letters Patent. Patented J 21 1132 Application filed February 10, 1920. fierial fio. 357,71'Y.

T 0 all whom it may concern:

Be it known that T, Jorrn LEONARD CLARKE, a subject of the King; of Great Britain, and residing at 366 l-lingston avenue, in the city and District of Montreal, in the Province of Quebec, in the Dominion of Canada, have invented a new and useful Electrical Measuring Apparatus, of which the following is the specification.

The invention relates to an electrical measuring apparatus as described in the present specification and illustrated in the accompanying drawings that form part of the same.

The invention consists essentially of an apparatus in which a series impedance is contained in one branch of an electrical circuit, while there is a gap in the other branch of the same circuit for the introduction of the line or part to be tested.

The objects of the'invention are to determine the transmission' losses occurring in transmission lines, especially where the flow of electrical current is very light, as in telephone, telegraph, signal and electrical.

distant indicator circuits, such transmis sion losses meaning the ch anges in the magnitude of the current and in the phase of the current as it passes from one point in the circuit to another; to furnish an apparatus that will measure the transmission losses at any frequency, suc as will he found in. the

' speech current in a telephone circuit, which is a complex wave built up of a number of waves having varying: frequencies; and genorally to provide a means for determining the losses or defect in lines ar parts making up telephone or other systems.

In the drawings, Figure 1 is a diagrammatic view, showing the wiring in a preferred form of the measuring apparatus.

Fig. 2 is a diagrammatic view showing a repeating coil inserted in the gap.

Fig. 3 is a'diagrammatic view, showing a retardation coil inserted in the gap.

Fig. 4 is'a diagrammatic view showing a transmission line in the gap.

Like characters of reference indicate corresponding parts in each figure.

The determination of" the transmission loss due to the insertion of any piece of apparatus, or circuit, in an electrical circuit, is equivalent to determining the vectorratio of the current existing in the circuit after the, apparatus is inserted in the circuit to the current existing in the circuit before the apparatus was inserted. It is assumed that the electromotive force producing the current is net affected by the change in the circuit produced by the insertion ofthe-apparatus.

If T, betaken as a vector quantity expressing the current in the circuit before the apparatus is inserted and 1 be taken as a vector quantity expressing" the current in the circuit after the apparatus is inserted, then 2 L in the above mentioned circuit due to the insertion of. the apparatus.

. This ratio may he enpresser ash: A 6where K is a scalar quantity equal to the ratio of the magnitudes of i and l, and L6 is the dilierence between the phase angle of T and the phase angle of T The value ofK may be transformed into miles ofstandard cable by means of the equation where L is a figure expressing miles of standard cable and o is an exponent depend ing on the frequency and the constants of the standard. cable or by means of curves graphically delineating the relations expressed by the above equation.

The ineasurementof this vector ratio K10 is accomplished lay-means of the apparatus described below. more fully understood by reference to Fig.

the ratio 1. where a detailed diagram of the circuit connections of one form of testing setis shown. This diagram shows a circuit having two branches A. and B each terminating in an impedance, the impedances are designated H and Branch A. contains a variable series impedance oesignated by the Figs. 1, 2 and 3 in the diagram. 1 is variable capacity, 2 is avariahle inductance, 3 isa variable resistance. 10 is a receiver or other means for detecting a potential difference between the points T and T 8 represents a break in circuit B terminated by the four terminals X X and Between these four terminals the apparatus of which the transmission loss isto be measured is connected. 9 is a reversing switch to be used when the angle of the series impedance in the A branch must be over 90 in order to give a balance in the receiver.

The networks 6 and 7 are so constructed that theimpedance across S S looking to ward E, and the impedance across 8,, S

This apparatus will he i looking toward E, are each made equal to the characteristic impedance of the circuit in which the apparatus whose-loss is to be measured is to' be used. 11 is a generator of A. G. current of the frequency at which it is desired to measure the loss in the apparatus. 12, 13 and 14, are double pole double throw switches for connecting in or removing out of the circuit A, the impedances 1,2 and 3. r

lhe testing set is operated as follows 1+ 'The'apparatus whose loss is to be ncas ured is connectedto the terminals X X X; if the apparatus is a repeating coil the primary terminals of the coil are connected to X and X and the secondary terminals to and X as in Fig. 2. If the apparatus is similar to a retardation coil which is normally bridged across the cir cuit, the terminals ofthecoil are connected 7 to X and X and'X is connected to X zero potentia Then if no potential exists B in the manner in which it and X isconnected to X 7 as in Fig. 3. Any apparatus is connected tothe circuit would be normally connected to the circuit in which it is toop'erate.

The impedances R R R and are adjusted so as to be each equal to the char: acteristic impedance of the circuit in which the loss in the apparatus is to be measured.

The source of A. C. current is connected to the terminals marked E and E and adjustments are made in the impedances' v1, 2 and 3 and if necessary reversing the switch 9 until no sound is heard in the receiver 10 or until the device for indicating potenbetween T and T no current is flowing n from i and T and since T and T are at the same potential this indicates that V the potential betwe'enl ld and T is. equal to the'potential 'between"T and T and since theimpedance R is equal to the impedance R it follows that the current in ll cis equal in magnitude to and is in phase with the current in R Now the current in branch will be p 7 R ii 34 Z where Z is the total'impedance due to the impedances 1, 2 and '3,- hencethe current in branch B is equal also to V i R R Z since R is in series with the circuit Band the currents in R and R are equal in'magintude and phase. r

Nowwhen X isconnected to X and X is connected to X; and when thefswitches 12, 13 and 14 are thrown downward to'cut out the impedances 1, 2 and'S, then the impedance 1n the circuit A is R +Rf and menses the impedance in the circuit B is R d-R but these nnpedances are all equal, hence the current in branch A is equal to the current in branch B and each will be equal to ing the apparatu in the circuit 13 to the current before inserting. the apparatus 111 circuit R will be equal to R +R +Z this then is equal to the transmission loss cue to inserting the apparatus in a circuit having; a characteristic impedance equal to This factor is equal to 1 when the switch 9 has been reversed then 10 is changed by 180.

diyidedby ilence the ratio ofthe current after insert- 7 When measuring the losses in circuits it ingy; manner. Let

1 V of the circuit with the repeating coils in be the loss series it h the circuit and v measured of the repeating cOllS in without the circuit, then the loss in the circuit l Will be divided-by See I1 I1 I1 Fig.4.

repeating coils which are normally used to connect together circuits of difierent impedances, each side ofthe coil is connected to suitable impedances, that is R5, and it will be different impedances butclil will be equal to R and R will be equal to R Owing to the method of balancing-the currents in the two branchesoi the bridge, it is possible to obtain 'neasurements ofthe transmission loss in piecesoi apparatus with an accuracy of.001 of a mile of standard cable. For obtaining ery accurate measurement it is necessary that the impedances R and R 'must be very closely adjusted to have the. saineimpedance it is also necessary that R be equal to R but the same degree ofexactness of balance is'not ,re' quired as between R 'and R It is also necessary that the source of A. 0. current In this invention I do not confine the be the loss measured in measuringthe loss in inequality ratio means of detecting the balance of the currents in R and It, to the method shown in the diagram, but any means may be used which does not allow the impedance terminating circuit 13 to be altered by the apparatus used to detect the difference in the currents in R and R, and similarly does not allow the impedance terminating circuit A to be altered in any way.

The variable capacity, the variable inductance and the variable resistance are made with indicating scales from which the transmission losses may be computed as or;- plained above.

What I claim is 1. In an electrical measuring apparatus, an electrical circuit having two branches, each terminating in an impedance and having therebetween a break and means for determining the condition of one branch in respect to the other, one of said branches having a gap wherein the line or apparatus is inserted for test and electrical means in the other b anch for making the current in the two branches equal in magnitude and in phase.

2. In an electrical measuring apparatus, an electrical circuit having two branches, one of said branches having a gap for the introduction of the line or apparatus to be tested, means for detecting the difierence in the magnitude and phase 01" the currents in the two branches, and means introduced in the other branch for varying the magnitude and phase of the current in it, and from the indications supplied by this means to determine the losses produced by the insertion of the line or apparatus inthe gap as aforesaid.

3. In a transmission measuring apparatus for determining transmission losses in electrical circuits, comprising a circuit having two branches, impedances to terminate each end of each of the branches in such a way as to render these terminating impedances each equal to the characteristic impedance of the circuit in which the apparatus is nor mally used and to simulate thereby the normal condition of use and allow the measurement of the transmission loss to be made in such a manner as to give a value equivalent to the value it would have under the conditions in which the apparatus is used 4:. In a transmission measuring apparatus for determining transmission losses in electrical circuits comprising a circuit having two branches, impedances to terminate each end oi each of the branches, means to vary these impedances so that they will simulate the impedances of the circuitin which the apparatus is normally used, and to iacllitate the measurement of transmission losses in the apparatus under different conditions of use.

5. In a transmissison measuring apparatus for determining transmission losses in electrical circuits, comprising a circuit having two branches, one of which contains a series impedance,means for varying the impedance in order to change the magnitude and phase of the current in the first branch, so that the eiiect of placing this impedance in series with this branch is to change the current in this branch in exactly the same manner as the change in the current in the second branch produced by inserting the apparatus in series with this second branch, and means for detecting a difierence in magnitude or in phase between the current in the first branch and the current in this second branch.

Signed at the city of Montreal, Quebec, Canada, this ethday of February, 1920.

J. L. CLARKE. 

